I do not use PayPal Website Payments Pro so I cannot tell you how it works in general; but I can point you to information concerning its integration iwth ShopSite. The API is built-in so all you need to do is select it in ShopSite and configure it by filling in the information ShopSite requires to set it up. Help documentation can be found here:
http://www.shopsite.com/help/10.1/en-US ... alpro.html
Also, there is a video tutorial on our website which shows the difference in the PayPal integrations:
http://www.shopsite.com/tutorial/PayPal.htm
-ShopSite Nicole